Add stpcpy(), and actually implement syslinux_version() and
syslinux_derivative_info().

Add zopen(), zfopen(), and zloadfile() to transparently open and
uncompress a gzip file (adding support for other formats is quite
trivial.

Once a file handle or file pointer is received, it can be treated like
any other one (fstat will report it as a socket, since the length is
unknown.)

gcc complains that fclose() is too big to be inlined with -Os, so
humour it and move fclose() out of line.

Actually implement strnlen(), which was in the header files but not in
the library.

Actually implement the library function syslinux_ipappend_strings(),
which had been in the header files but not in the actual library.

Make pixel formatting an integral part of __vesacon_copy_to_screen().
In order to cut down on the dispatch overhead, make the formatting
functions contain the pixel loop, so we only do indirect dispatch
twice per pixel row - once for the formatting and once for copying to
the screen.

Add support for paged ("banked", non-linear-framebuffer) VESA video
modes.  Apparently some manufacturers haven't caught on that
non-linear graphics modes have been obsolete for 15 years or so
already.

Initial support for the auxillary data vector, a storage mechanism
intended to support boot-once and similar features.